Climate and tick seasonality are predictors of Borrelia burgdorferi genotype distribution

Impact of climate trends on tick-borne pathogen transmission

Recent advances in climate research together with a better understanding of tick-pathogen interactions, the distribution of ticks and the diagnosis of tick-borne pathogens raise questions about the impact of environmental factors on tick abundance and spread and the prevalence and transmission of tick-borne pathogens. While undoubtedly climate plays a role in the changes in distribution and seasonal abundance of ticks, it is always difficult to disentangle factors impacting on the abundance of tick hosts from those exerted by human habits. All together, climate, host abundance, and social factors may explain the upsurge of epidemics transmitted by ticks to humans. Herein we focused on tick-borne pathogens that affect humans with epidemic potential. Borrelia burgdorferi s.l. (Lyme disease), Anaplasma phagocytophilum (human granulocytic anaplasmosis), and tick-borne encephalitis virus (tick-borne encephalitis) are transmitted by Ixodes spp. Crimean-Congo hemorrhagic fever virus (Crimean-Congo hemorrhagic fever) is transmitted by Hyalomma spp. In this review, we discussed how vector tick species occupy the habitat as a function of different climatic factors, and how these factors impact on tick survival and seasonality. How molecular events at the tick-pathogen interface impact on pathogen transmission is also discussed. Results from statistically and biologically derived models are compared to show that while statistical models are able to outline basic information about tick distributions, biologically derived models are necessary to evaluate pathogen transmission rates and understand the effect of climatic variables and host abundance patterns on pathogen transmission. The results of these studies could be used to build early alert systems able to identify the main factors driving the subtle changes in tick distribution and seasonality and the prevalence of tick-borne pathogens.

A spatially-explicit model of acarological risk of exposure to Borrelia burgdorferi-infected Ixodes pacificus nymphs in northwestern California based on woodland type, temperature, and water vapor

In the far-western United States, the nymphal stage of the western black-legged tick, Ixodes pacificus, has been implicated as the primary vector to humans of Borrelia burgdorferi sensu stricto (hereinafter referred to as B. burgdorferi), the causative agent of Lyme borreliosis in North America. In the present study, we sought to determine if infection prevalence with B. burgdorferi in I. pacificus nymphs and the density of infected nymphs differ between dense-woodland types within Mendocino County, California, and to develop and evaluate a spatially-explicit model for density of infected nymphs in dense woodlands within this high-incidence area for Lyme borreliosis. In total, 4.9% (264) of 5431 I. pacificus nymphs tested for the presence of B. burgdorferi were infected. Among the 78 sampling sites, infection prevalence ranged from 0 to 22% and density of infected nymphs from 0 to 2.04 per 100 m(2). Infection prevalence was highest in woodlands dominated by hardwoods (6.2%) and lowest for redwood (1.9%) and coastal pine (0%). Density of infected nymphs also was higher in hardwood-dominated woodlands than in conifer-dominated ones that included redwood or pine. Our spatial risk model, which yielded an overall accuracy of 85%, indicated that warmer areas with less variation between maximum and minimum monthly water vapor in the air were more likely to include woodlands with elevated acarological risk of exposure to infected nymphs. We found that 37% of dense woodlands in the county were predicted to pose an elevated risk of exposure to infected nymphs, and that 94% of the dense-woodland areas that were predicted to harbor elevated densities of infected nymphs were located on privately-owned land.

Genotypic diversity of an emergent population of Borrelia burgdorferi at a coastal Maine island recently colonized by Ixodes scapularis

The recent range expansion of Ixodes scapularis has been accompanied by the emergence of Borrelia burgdorferi. The development of genetic diversity in B. burgdorferi at these sites of emergence and its relationship to range expansion is poorly understood. We followed colonization of I. scapularis on a coastal Maine island over a 17-year period. B. burgdorferi's emergence was documented, as was expansion of ospC strain diversity. Ticks collected from rodents and vegetation were examined for the presence of B. burgdorferi. Sequencing and reverse line blot were used to detect B. burgdorferi ospC major groups (oMG). No I. scapularis were found until year four of the study, after which time they increased in abundance. No B. burgdorferi was detected by darkfield microscopy in I. scapularis until 10 years into the study, when 4% of adult ticks were infected. Seven years later, 43% of adult ticks were infected. In 2003, one oMG accounted for 91% of B. burgdorferi strains. This "founder" strain persisted in 2005, but by 2007 was a minority of the 7 oMGs present. Given the island's isolation, gene flow by avian introduction of multiple strains is suggested in the development of B. burgdorferi oMG diversity.

What do we need to know about disease ecology to prevent Lyme disease in the northeastern United States?

Lyme disease is the most commonly reported vector-borne disease in the United States, with the majority of cases occurring in the Northeast. It has now been three decades since the etiological agent of the disease in North America, the spirochete Borrelia burgdorferi, and its primary North American vectors, the ticks Ixodes scapularis Say and I. pacificus Cooley & Kohls, were identified. Great strides have been made in our understanding of the ecology of the vectors and disease agent, and this knowledge has been used to design a wide range of prevention and control strategies. However, despite these advances, the number of Lyme disease cases have steadily increased. In this article, we assess potential reasons for the continued lack of success in prevention and control of Lyme disease in the northeastern United States, and identify conceptual areas where additional knowledge could be used to improve Lyme disease prevention and control strategies. Some of these areas include: 1) identifying critical host infestation rates required to maintain enzootic transmission of B. burgdorferi, 2) understanding how habitat diversity and forest fragmentation impacts acarological risk of exposure to B. burgdorferi and the ability of interventions to reduce risk, 3) quantifying the epidemiological outcomes of interventions focusing on ticks or vertebrate reservoirs, and 4) refining knowledge of how human behavior influences Lyme disease risk and identifying barriers to the adoption of personal protective measures and environmental tick management.

Multilocus sequence analysis of Borrelia bissettii strains from North America reveals a new Borrelia species, Borrelia kurtenbachii

Using multilocus sequence analyses (MLSA), we investigated the phylogenetic relationship of spirochaete strains from North America previously assigned to the genospecies Borrelia bissettii. We amplified internal fragments of 8 housekeeping genes (clpA, clpX, nifS, pepX, pyrG, recG, rplB, and uvrA) located on the main linear chromosome by polymerase chain reaction. Phylogenetic analysis of concatenated sequences of the 8 loci showed that the B. bissettii clade consisted of 4 closely related clusters which included strains from California (including the type strain DN127-Cl9-2/p7) and Colorado that were isolated from Ixodes pacificus, I. spinipalpis, or infected reservoir hosts. Several strains isolated from I. scapularis clustered distantly from B. bissettii. Genetic distance analyses confirmed that these strains are more distant to B. bissettii than they are to B. carolinensis, a recently described Borrelia species, which suggests that they constitute a new Borrelia genospecies. We propose that it be named Borrelia kurtenbachii sp. nov. in honour of the late Klaus Kurtenbach. The data suggest that ecological differences between B. bissettii and the new Borrelia genospecies reflect different transmission cycles. In view of these findings, the distinct vertebrate host-tick vector associations and the distributions of B. bissettii and B. kurtenbachii require further investigation.

Associations between Ixodes scapularis ticks and small mammal hosts in a newly endemic zone in southeastern Canada: implications for Borrelia burgdorferi transmission

Immature Ixodes scapularis infestation and Borrelia burgdorferi infection of wild small mammals were studied from June to October in 2007 and from May to October in 2008 at 71 study sites in a zone where I. scapularis populations and environmental Lyme disease risk are emerging in southwestern Quebec. Seasonal host-seeking activity of immature I. scapularis was similar to patterns reported previously in Canada and the USA: nymphal activity peaked in spring while larval activity peaked in late summer. Synchronous activity of nymphs with some larvae was observed in late spring, which could favour establishment of B. burgdorferi strains that cause short-lived infections in their hosts. White-footed mice (Peromyscus leucopus), deer mice (P. maniculatus), chipmunks (Tamias striatus), and red squirrels (Tamiasciurus hudsonicus) carried 92.0% of the larvae and 94.2% of the nymphs collected. Adult male white-footed mice carried significantly larger numbers of both larval and nymphal I. scapularis than other species and classes of small mammals (different demographic groups or physiological status: age, sex, sexual activity). We conclude that seasonality and host association were comparable to previous studies in North America, even in the context of a newly endemic pattern of low infection prevalence and low densities of host-seeking and feeding I. scapularis in southwestern Quebec. Our studies suggest that B. burgdorferi transmission cycles are focused on adult male mice (which carried 35% of all feeding ticks collected in the study), so control methods targeting this class of hosts may be particularly effective. However, our study also suggested that habitats containing a diverse host structure may dilute transmission cycles by partitioning of nymphal and larval ticks on different host species.

Agents of human anaplasmosis and Lyme disease at Camp Ripley, Minnesota

The transmission dynamics of Anaplasma phagocytophilum (Ap) and Borrelia burgdorferi (Bb) among Ixodes scapularis (Is) and mammalian hosts was investigated at Camp Ripley, an area representative of central Minnesota. Prevalence of white-footed mouse infection with Ap and Bb were 20% and 42%, respectively, with a coinfection level of 14%. Peak levels of infection with both agents occurred in May. The average levels of seropositivity to Ap and Bb were 29.3% and 48%, respectively. Of the mice infected with Ap, 47.5% were able to eliminate the pathogen as compared with 19.4% of mice infected with Bb. Ap was detected in 88.4% of 43 eastern chipmunks examined and isolated from 44.7% of the animals. Bb was present in 72.7% of 11 chipmunks examined, and 100% of the animals were also infected with Ap. The seasonality of tick activity differs from that reported for the New York area. Is infestation of mice began in May with peak nymphal infestation also occurring in May (7.4 per infested mouse) and overlapping with peak larval infestation in June (77.1 per infested mouse). Infestation ranged from 100% in May to 34.5% in October. Is comprised 98.4% of the ticks infesting the mice. The temporal pattern of the developmental stages of Is infesting chipmunks was the same as for mice, except that the tick burdens were greater. The nymphal stage peaked in May (81.3 per animal), and the larval stage peaked in June (164.7 per animal). Infestation was 100% in May-August, and >99% of the ticks were Is. Antibodies to Ap were present in >80% of the white-tailed deer examined, but they were infected with the Ap-1 variant rather than the Ap strain infecting mice and humans. Antibodies to Bb were detected in >80% of the deer, but Bb DNA was only detected in 1.5% of blood specimens.

Borrelia afzelii ospC genotype diversity in Ixodes ricinus questing ticks and ticks from rodents in two Lyme borreliosis endemic areas: contribution of co-feeding ticks

In Europe, the Lyme borreliosis (LB) agents like Borrelia burgdorferi sensu stricto (ss), B. afzelii, and B. garinii are maintained in nature by enzoonotic transmission cycles between vertebrate hosts and Ixodes ricinus ticks. The outer surface protein C is a highly antigenic protein expressed by spirochaetes during transmission from ticks to mammals as well as during dissemination in the vertebrate hosts. Previous studies based on analysis of ospC gene sequences have led to the classification of ospC genotypes into ospC groups. The aim of this study was to analyse and compare ospC group distribution among isolates of the rodent-associated genospecies, B. afzelii, at 3 levels (questing ticks, ticks feeding on rodents, and xenodiagnostic ticks). Isolates were obtained during a study carried out in 2 LB endemic areas located on the Swiss Plateau [Portes-Rouges (PR) and Staatswald (SW)], where rodents were differently infested by co-feeding ticks (Pérez et al., unpublished data). Overall, we identified 10 different ospC groups with different distributions among isolates from questing ticks, ticks that detached from rodents, and xenodiagnostic ticks at the 2 sites. We observed a higher ospC diversity among isolates from ticks that fed on rodents at SW, and mixed infections with 2 ospC groups were also more frequent among isolates from ticks that fed on rodents at SW (n=18) than at PR (n=1). At both sites, B. afzelii isolates obtained from larvae that were feeding on the rodents simultaneously with nymphs displayed a higher diversity of ospC groups (mean number of ospC groups: 2.25 for PR and 1.75 for SW) than isolates from larvae feeding without nymphs (mean number of ospC groups: 1.17 for PR and 1 for SW). We suggest that co-feeding transmission of Borrelia, previously described in laboratory models, contributes in nature in promoting and maintaining ospC diversity within local tick populations.

Population genetics, taxonomy, phylogeny and evolution of Borrelia burgdorferi sensu lato

In order to understand the population structure and dynamics of bacterial microorganisms, typing systems that accurately reflect the phylogenetic and evolutionary relationship of the agents are required. Over the past 15 years multilocus sequence typing schemes have replaced single locus approaches, giving novel insights into phylogenetic and evolutionary relationships of many bacterial species and facilitating taxonomy. Since 2004, several schemes using multiple loci have been developed to better understand the taxonomy, phylogeny and evolution of Lyme borreliosis spirochetes and in this paper we have reviewed and summarized the progress that has been made for this important group of vector-borne zoonotic bacteria.

Genetic diversity in Ixodes scapularis (Acari: Ixodidae) from six established populations in Canada

Although Ixodes scapularis is the most important vector of Borrelia burgdorferi sensu stricto, the causative agent of Lyme disease in North America, little is known of the genetic diversity in this tick species within the recently established populations in Canada. In the present study, 153 I. scapularis adults collected from southern Canada were compared genetically using single-strand conformation polymorphism (SSCP) analyses in combination with DNA sequencing of the mitochondrial 16S rRNA gene. Nineteen haplotypes were detected, 8 of which have not been reported in the U.S.A. One 'new' haplotype was only detected at Lunenburg, Nova Scotia, and comprised 38% of the ticks examined for that population. The population in the southeast corner of Manitoba contained 3 'new' haplotypes. Although the most common haplotype (Is-1) was present in all 6 populations of I. scapularis in Canada, there were significant differences in the genetic structure among population. This suggests different geographical origins for the tick populations in Canada, which may be related to the transportation of larval and nymphal ticks by migratory passerines using different flyways. Determination of the origins of the endemic populations of I. scapularis in Canada, as well as those predicted to establish in the near future, has important implications with respect to understanding the risk of exposure to pathogenic bacteria infecting these ticks.

Correlation of Borrelia burgdorferi sensu lato prevalence in questing Ixodes ricinus ticks with specific abiotic traits in the western palearctic

This meta-analysis of reports examining ticks throughout the Western Palearctic region indicates a distinct geographic pattern for Borrelia burgdorferi sensu lato prevalence in questing nymphal Ixodes ricinus ticks. The greatest prevalence was reported between the 5°E and 25°E longitudes based on an analysis of 123 collection points with 37,940 nymphal tick specimens (87.43% of total nymphs; 56.35% of total ticks in the set of reports over the target area). Climatic traits, such as temperature and vegetation stress, and their seasonality correlated with Borrelia prevalence in questing ticks. The greatest prevalence was associated with mild winter, high summer, and low seasonal amplitude of temperatures within the range of the tick vector, higher vegetation indices in the May-June period, and well-connected vegetation patches below a threshold at which rates suddenly drop. Classification of the target territory using a qualitative risk index derived from the abiotic variables produced an indicator of the probability of finding infected ticks in the Western Palearctic region. No specific temporal trends were detected in the reported prevalence. The ranges of the different B. burgdorferi sensu lato genospecies showed a pattern of high biodiversity between 4°W and 20°E, partially overlapping the area of highest prevalence in ticks. Borrelia afzelii and Borrelia garinii are the dominant species in central Europe (east of ∼25°E), but B. garinii may appear alone at southern latitudes and Borrelia lusitaniae is the main indicator species for meridional territories.

Uncoordinated phylogeography of Borrelia burgdorferi and its tick vector, Ixodes scapularis

Vector-borne microbes necessarily co-occur with their hosts and vectors, but the degree to which they share common evolutionary or biogeographic histories remains unexplored. We examine the congruity of the evolutionary and biogeographic histories of the bacterium and vector of the Lyme disease system, the most prevalent vector-borne disease in North America. In the eastern and midwestern US, Ixodes scapularis ticks are the primary vectors of Borrelia burgdorferi, the bacterium that causes Lyme disease. Our phylogeographic and demographic analyses of the 16S mitochondrial rDNA suggest that northern I. scapularis populations originated from very few migrants from the southeastern US that expanded rapidly in the Northeast and subsequently in the Midwest after the recession of the Pleistocene ice sheets. Despite this historical gene flow, current tick migration is restricted even between proximal sites within regions. In contrast, B. burgdorferi suffers no barriers to gene flow within the northeastern and midwestern regions but shows clear interregional migration barriers. Despite the intimate association of B. burgdorferi and I. scapularis, the population structure, evolutionary history, and historical biogeography of the pathogen are all contrary to its arthropod vector. In the case of Lyme disease, movements of infected vertebrate hosts may play a larger role in the contemporary expansion and homogenization of the pathogen than the movement of tick vectors whose populations continue to bear the historical signature of climate-induced range shifts.

Diverse Borrelia burgdorferi strains in a bird-tick cryptic cycle

The blacklegged tick Ixodes scapularis is the primary vector of the most prevalent vector-borne zoonosis in North America, Lyme disease (LD). Enzootic maintenance of the pathogen Borrelia burgdorferi by I. scapularis and small mammals is well documented, whereas its "cryptic" maintenance by other specialist ticks and wildlife hosts remains largely unexplored because these ticks rarely bite humans. We quantified B. burgdorferi infection in a cryptic bird-rabbit-tick cycle. Furthermore, we explored the role of birds in maintaining and moving B. burgdorferi strains by comparing their genetic diversity in this cryptic cycle to that found in cycles vectored by I. scapularis. We examined birds, rabbits, and small mammals for ticks and infection over a 4-year period at a focal site in Michigan, 90 km east of a zone of I. scapularis invasion. We mist netted 19,631 birds that yielded 12,301 ticks, of which 86% were I. dentatus, a bird-rabbit specialist. No resident wildlife harbored I. scapularis, and yet 3.5% of bird-derived ticks, 3.6% of rabbit-derived ticks, and 20% of rabbit ear biopsy specimens were infected with B. burgdorferi. We identified 25 closely related B. burgdorferi strains using an rRNA gene intergenic spacer marker, the majority (68%) of which had not been reported previously. The presence of strains common to both cryptic and endemic cycles strongly implies bird-mediated dispersal. Given continued large-scale expansion of I. scapularis populations, we predict that its invasion into zones of cryptic transmission will allow for bridging of novel pathogen strains to humans and animals.

Regional variation in immature Ixodes scapularis parasitism on North American songbirds: implications for transmission of the Lyme pathogen, Borrelia burgdorferi

Borrelia burgdorferi, the etiological agent of Lyme disease, is transmitted among hosts by the black-legged tick, Ixodes scapularis, a species that regularly parasitizes various vertebrate hosts, including birds, in its immature stages. Lyme disease risk in the United States is highest in the Northeast and in the upper Midwest where I. scapularis ticks are most abundant. Because birds might be important to the range expansion of I. scapularis and B. burgdorferi, we explored spatial variation in patterns of I. scapularis parasitism on songbirds, as well as B. burgdorferi infection in bird-derived I. scapularis larvae. We sampled birds at 23 sites in the eastern United States to describe seasonal patterns of I. scapularis occurrence on birds, and we screened a subset of I. scapularis larvae for presence of B. burgdorferi. Timing of immature I. scapularis occurrence on birds is consistent with regional variation in host-seeking activity with a generally earlier peak in larval parasitism on birds in the Midwest. Significantly more I. scapularis larvae occurred on birds that were contemporaneously parasitized by nymphs in the Midwest than the Northeast, and the proportion of birds that yielded B. burgdorferi-infected larvae was also higher in the Midwest. We conclude that regional variation in immature I. scapularis phenology results in different temporal patterns of parasitism on birds, potentially resulting in differential importance of birds to B. burgdorferi transmission dynamics among regions.

Using geographic information systems and decision support systems for the prediction, prevention, and control of vector-borne diseases

Emerging and resurging vector-borne diseases cause significant morbidity and mortality, especially in the developing world. We focus on how advances in mapping, Geographic Information System, and Decision Support System technologies, and progress in spatial and space-time modeling, can be harnessed to prevent and control these diseases. Major themes, which are addressed using examples from tick-borne Lyme borreliosis; flea-borne plague; and mosquito-borne dengue, malaria, and West Nile virus disease, include (a) selection of spatial and space-time modeling techniques, (b) importance of using high-quality and biologically or epidemiologically relevant data, (c) incorporation of new technologies into operational vector and disease control programs, (d) transfer of map-based information to stakeholders, and (e) adaptation of technology solutions for use in resource-poor environments. We see great potential for the use of new technologies and approaches to more effectively target limited surveillance, prevention, and control resources and to reduce vector-borne and other infectious diseases.

Genotypic diversity of Borrelia burgdorferi strains detected in Ixodes scapularis larvae collected from North American songbirds

We genotyped Borrelia burgdorferi strains detected in larvae of Ixodes scapularis removed from songbirds and compared them with those found in host-seeking I. scapularis nymphs sampled throughout the eastern United States. Birds are capable of transmitting most known genotypes, albeit at different frequencies than expected based on genotypes found among host-seeking nymphs.

Low-density microarrays for the detection of Borrelia burgdorferi s.s. (the Lyme disease spirochete) in nymphal Ixodes scapularis

Lyme disease is the most common tick-borne disease in Europe and North America. In the hyperendemic Lyme disease regions of the eastern United States, nymphal Ixodes scapularis are the principal ticks transmitting the Lyme disease spirochete, Borrelia burgdorferi sensu stricto (s.s.). Approximately 25% of questing nymphs in endemic regions are infected with spirochetes. High throughput-sensitive and specific methods for testing nymphal I. scapularis for infection with B. burgdorferi are clearly needed. In the current study, we evaluated whether low-density microarrays could be adapted for the rapid and accurate detection and characterization of spirochetes in nymphal I. scapularis. Three different microarray platforms were developed and tested for the detection of spirochetes in ticks. They could both detect and differentiate different Borrelia genospecies, in one case detecting as few as a single copy of Borrelia DNA.

Loop analysis for pathogens: niche partitioning in the transmission graph for pathogens of the North American tick Ixodes scapularis

In population biology, loop analysis is a method of decomposing a life cycle graph into life history pathways so as to compare the relative contributions of pathways to the population growth rate across species and populations. We apply loop analysis to the transmission graph of five pathogens known to infect the black-legged tick, Ixodes scapularis. In this context loops represent repeating chains of transmission that could maintain the pathogen. They hence represent completions of the life cycle, in much the same way as loops in a life cycle graph do for plants and animals. The loop analysis suggests the five pathogens fall into two distinct groups. Borellia burgdorferi, Babesia microti and Anaplasma phagocytophilum rely almost exclusively on a single loop representing transmission to susceptible larvae feeding on vertebrate hosts that were infected by nymphs. Borellia miyamotoi, in contrast, circulates among a separate set of host types and utilizes loops that are a mix of vertical transmission and horizontal transmission. For B. miyamotoi the main loop is from vertebrate hosts to susceptible nymphs, where the vertebrate hosts were infected by larvae that were infected from birth. The results for Powassan virus are similar to B. miyamotoi. The predicted impacts of the known variation in tick phenology between populations of I. scapularis in the Midwest and Northeast of the United States are hence markedly different for the two groups. All of these pathogens benefit, though, from synchronous activity of larvae and nymphs.

Evolution and distribution of the ospC Gene, a transferable serotype determinant of Borrelia burgdorferi

Borrelia burgdorferi, an emerging bacterial pathogen, is maintained in nature by transmission from one vertebrate host to another by ticks. One of the few antigens against which mammals develop protective immunity is the highly polymorphic OspC protein, encoded by the ospC gene on the cp26 plasmid. Intragenic recombination among ospC genes is known, but the extent to which recombination extended beyond the ospC locus itself is undefined. We accessed and supplemented collections of DNA sequences of ospC and other loci from ticks in three U.S. regions (the Northeast, the Midwest, and northern California); a total of 839 ospC sequences were analyzed. Three overlapping but distinct populations of B. burgdorferi corresponded to the geographic regions. In addition, we sequenced 99 ospC flanking sequences from different lineages and compared the complete cp26 sequences of 11 strains as well as the cp26 bbb02 loci of 56 samples. Besides recombinations with traces limited to the ospC gene itself, there was evidence of lateral gene transfers that involved (i) part of the ospC gene and one of the two flanks or (ii) the entire ospC gene and different lengths of both flanks. Lateral gene transfers resulted in different linkages between the ospC gene and loci of the chromosome or other plasmids. By acquisition of the complete part or a large part of a novel ospC gene, an otherwise adapted strain would assume a new serotypic identity, thereby being comparatively fitter in an area with a high prevalence of immunity to existing OspC types.

The tick-borne zoonosis Lyme borreliosis is increasing in incidence and spreading geospatially in North America. Further understanding of the evolution and genetics of its cause, Borrelia burgdorferi, in its environments fosters progress toward ecologically based control efforts. By means of DNA sequencing of a large sample collection of the pathogen from across the United States, we studied the gene for the bacterium’s highly diverse OspC protein, protective immunity against which develops in animals. We found that the distributions and frequencies of types of OspC genes differed between populations of B. burgdorferi in the Northeast, the Midwest, and California. Over time, OspC genes were transferred between strains through recombinations involving the whole or parts of the gene and one or both flanks. Acquisitions of OspC genes that are novel for the region confer to recipients unique identities to host immune systems and, presumably, selective advantage when immunity to existing types is widespread among hosts.

Invasion of the lyme disease vector Ixodes scapularis: implications for Borrelia burgdorferi endemicity

Lyme disease risk is increasing in the United States due in part to the spread of blacklegged ticks Ixodes scapularis, the principal vector of the spirochetal pathogen Borrelia burgdorferi. A 5-year study was undertaken to investigate hypothesized coinvasion of I. scapularis and B. burgdorferi in Lower Michigan. We tracked the spatial and temporal dynamics of the tick and spirochete using mammal, bird, and vegetation drag sampling at eight field sites along coastal and inland transects originating in a zone of recent I. scapularis establishment. We document northward invasion of these ticks along Michigan's west coast during the study period; this pattern was most evident in ticks removed from rodents. B. burgdorferi infection prevalences in I. scapularis sampled from vegetation in the invasion zone were 9.3% and 36.6% in nymphs and adults, respectively, with the majority of infection (95.1%) found at the most endemic site. There was no evidence of I. scapularis invasion along the inland transect; however, low-prevalence B. burgdorferi infection was detected in other tick species and in wildlife at inland sites, and at northern coastal sites in years before the arrival of I. scapularis. These infections suggest that cryptic B. burgdorferi transmission by other vector-competent tick species is occurring in the absence of I. scapularis. Other Borrelia spirochetes, including those that group with B. miyamotoi and B. andersonii, were present at a low prevalence within invading ticks and local wildlife. Reports of Lyme disease have increased significantly in the invasion zone in recent years. This rapid blacklegged tick invasion--measurable within 5 years--in combination with cryptic pathogen maintenance suggests a complex ecology of Lyme disease emergence in which wildlife sentinels can provide an early warning of disease emergence.

Evolution of northeastern and midwestern Borrelia burgdorferi, United States

Differences in animal ecology or human behavior may account for differences in human incidence in the 2 regions.

The per capita incidence of human Lyme disease in the northeastern United States is more than twice that in the Midwest. However, the prevalence of Borrelia burgdorferi, the bacterium that causes Lyme disease, in the tick vector is nearly identical in the 2 regions. The disparity in human Lyme disease incidence may result from a disparity in the human invasiveness of the bacteria in the Northeast and Midwest caused by fundamentally different evolutionary histories. B. burgdorferi populations in the Northeast and Midwest are geographically isolated, enabling evolutionary divergence in human invasiveness. However, we found that B. burgdorferi populations in the Northeast and Midwest shared a recent common ancestor, which suggests that substantial evolutionary divergence in human invasiveness has not occurred. We propose that differences in either animal ecology or human behavior are the root cause of the differences in human incidence between the 2 regions.

Niche partitioning of Borrelia burgdorferi and Borrelia miyamotoi in the same tick vector and mammalian reservoir species

The Lyme borreliosis agent Borrelia burgdorferi and the relapsing fever group species Borrelia miyamotoi co-occur in the United States. We used species-specific, quantitative polymerase chain reaction to study both species in the blood and skin of Peromyscus leucopus mice and host-seeking Ixodes scapularis nymphs at a Connecticut site. Bacteremias with B. burgdorferi or B. miyamotoi were most prevalent during periods of greatest activity for nymphs or larvae, respectively. Whereas B. burgdorferi was 30-fold more frequent than B. miyamotoi in skin biopsies and mice had higher densities of B. burgdorferi densities in the skin than in the blood, B. miyamotoi densities were higher in blood than skin. In a survey of host-seeking nymphs in 11 northern states, infection prevalences for B. burgdorferi and B. miyamotoi averaged approximately 0.20 and approximately 0.02, respectively. Co-infections of P. leucopus or I. scapularis with both B. burgdorferi and B. miyamotoi were neither more nor less common than random expectations.

Phylogeography of Borrelia burgdorferi in the eastern United States reflects multiple independent Lyme disease emergence events

Since its first description in coastal Connecticut in 1976, both the incidence of Lyme disease and the geographic extent of endemic areas in the US have increased dramatically. The rapid expansion of Lyme disease into its current distribution in the eastern half of the US has been due to the range expansion of the tick vector, Ixodes scapularis, upon which the causative agent, Borrelia burgdorferi is dependent for transmission to humans. In this study, we examined the phylogeographic population structure of B. burgdorferi throughout the range of I. scapularis-borne Lyme disease using multilocus sequence typing based on bacterial housekeeping genes. We show that B. burgdorferi populations from the Northeast and Midwest are genetically distinct, but phylogenetically related. Our findings provide strong evidence of prehistoric population size expansion and east-to-west radiation of descendent clones from founding sequence types in the Northeast. Estimates of the time scale of divergence of northeastern and midwestern populations suggest that B. burgdorferi was present in these regions of North America many thousands of years before European settlements. We conclude that B. burgdorferi populations have recently reemerged independently out of separate relict foci, where they have persisted since precolonial times.

Reviewing molecular adaptations of Lyme borreliosis spirochetes in the context of reproductive fitness in natural transmission cycles

Lyme borreliosis (LB) is caused by a group of pathogenic spirochetes – most often Borrelia burgdorferi, B. afzelii, and B. garinii – that are vectored by hard ticks in the Ixodes ricinus-persulcatus complex, which feed on a variety of mammals, birds, and lizards. Although LB is one of the best-studied vector-borne zoonoses, the annual incidence in North America and Europe leads other vector-borne diseases and continues to increase. What factors make the LB system so successful, and how can researchers hope to reduce disease risk – either through vaccinating humans or reducing the risk of contacting infected ticks in nature? Discoveries of molecular interactions involved in the transmission of LB spirochetes have accelerated recently, revealing complex interactions among the spirochete-tick-vertebrate triad. These interactions involve multiple, and often redundant, pathways that reflect the evolution of general and specific mechanisms by which the spirochetes survive and reproduce. Previous reviews have focused on the molecular interactions or population biology of the system. Here molecular interactions among the LB spirochete, its vector, and vertebrate hosts are reviewed in the context of natural maintenance cycles, which represent the ecological and evolutionary contexts that shape these interactions. This holistic system approach may help researchers develop additional testable hypotheses about transmission processes, interpret laboratory results, and guide development of future LB control measures and management.